The spaceflights undertaken to date demonstrated that Man can adapt to the conditions existing in space, particularly to weightlessness. However, the space environment and the prevailing micro-gravity induce modifications which may affect the performances of the astronauts. These modifications concern in particular the cardiovascular and hormonal systems, the muscles, the bones, the blood and the immune system. Other problems may occur during long duration flights, such as the problems created by psychological stress due to the confinement, or those resulting from the cosmic rays (radiation).